This invention relates to nuclear reactor fuel assemblies, and in particular to structure carried by the assemblies for reducing the magnitude of the lateral deformation, or bowing, of the assembly which may occur during reactor operation.
Each of approximately 200 fuel assemblies in a modern pressurized water reactor (PWR) typically consists of a square array of fuel rods having external dimensions on the order of 8.times.8.times.160 inches (20.times.20.times.410 centimeters). In order to maximize neutron economy, it is highly desirable to make all structural components of the assembly from Zircaloy. In contrast, the reactor vessel internal structures, which support the fuel assemblies, are typically made from type 304 stainless steel.
During reactor operation, forces acting on the fuel assemblies tend to cause small lateral distortions of the fuel assembly structures. The only upper limit on the total magnitude of such distortions is the summation of the lateral clearances between the fuel assemblies. Assemblies having all Zircaloy structures are more susceptible to such deformation than those having stainless or Inconel structures because Zircaloy has a lower elastic modulus and tends to creep under irradiated conditions at a greater rate than stainless steel or Inconel, thereby assuming a slightly bowed shape in less time than the duration of a typical reactor cycle. Such distortions are undesirable because they may complicate refueling and they may introduce slight variations in local power density by virtue of the uneven water gap between assemblies.
The magnitude of the nominal lateral clearance between adjacent fuel assemblies is determined by the outside dimensions of the fuel assembly grids. Compared with stainless steel or Inconel grids, Zircaloy grids have two distinct differences with respect to fuel assembly bowing. First, the initial clearance for the Zircaloy grid must include an allowance for irradiation induced lateral growth. Otherwise, clearances between irradiated assemblies will become so small that withdrawing and inserting individual assemblies during refueling may become difficult. Second, differential expansion between the stainless steel vessel internals structure and the Zircaloy grids causes the clearance at operating temperatures to increase substantially (up to 50 percent), thereby allowing space for larger bowing during operation.
It has been proposed to reduce the bowing by using one or more stainless steel or Inconel grids near the midplane of the assemblies. Although such a grid would limit bowing, it is not a desirable solution for two reasons. First, the replacement of even a single Zircaloy grid with one of stainless steel would increase parasitic neutron absorption. Second, the greater lateral stiffness of stainless steel grids relative to Zircaloy, coupled with the lower lateral clearance of the stainless grids, would cause impact loads associated with seismic disturbances or accident conditions such as loss of coolant, to be concentrated on the stainless grid, thereby necessitating an extremely strong grid.